Injection technique is playing an increasingly important part in the metallurgy of steel for developing continuously improved qualities of steel. By injecting fine particle size calcium and/or magnesium compounds or metallic calcium and/or magnesium it has thus been possible for example to reduce the total content of sulphur in steel melts to extremely low values, i.e. less than 50 ppm. By choosing appropriate injection products it has also been possible to transform residual sulphides so that these become as harmless as possible. With the aid of injection metallurgy it has also been possible to reduce the content of oxidic inclusions to levels which could only be achieved with difficulty using techniques hitherto traditional. However in the search for improved grades of steel the requirement for extremely low sulphur contents has been imposed, i.e. not exceeding 20 ppm, which using methods known hitherto could not be achieved with good reproducibility without a deterioration in quality requirements other than the low sulphur content.
The problem outlined above is due to the dynamics of reactions of the type which take place between on the one hand the ladle lining the furnace slag and the atmosphere above the steel bath, and secondly the steel melt and those substances with a high oxygen affinity which are dissolved in the steel melt, and to the oxygen activity in the melt which is dependent on these reactions. The reaction from the ladle lining can be minimized by selecting a suitable refractory material, but the reaction from furnace slag residuals and by contact with the air so far have been difficult to control with good reproducibility. Obviously it has been possible to a considerable extent to influence the conditions by partly replacing the acid and usually FeO- rich furnace slag with a synthetic slag of high basicity, and also by de-oxidising the steel melt before any desulphurization substance is injected. With the methods and the technical aids available hitherto however it has not been possible to achieve such high accuracy during operations as to achieve with good reproducibility such extremely low sulphur contents as circa 200 ppm max.